This invention relates to the interface between telecommunications equipment and a communication channel, e.g., the interface between a modem and a two wire telephone line.
Interfacing communications equipment to a two-line communication channel, such as the local loop of a phone line, where signals are to be transferred in both directions in the same frequency range generally requires a circuit called a hybrid. The hybrid performs the function of cancelling out the locally transmitted signal from the remotely transmitted signal. In modems, this circuit may take the form of a simple op-amp subtractor circuit.
Two issues that arise in designing hybrids are echoes and imperfect subtraction of the local transmitted signal, also known as immediate near-end echo. Echoes arise wherever there is a mismatch in impedance in the local loop, and thus the designer may reduce the amount of echo in the system by matching the impedance of the hybrid to that of the local loop.
One factor that may cause imperfect subtraction of the local transmitted signal is an incorrect matching of the input and feedback impedances around the op-amp in the subtractor circuit. These impedances, as is well known in the art, determine the gain of signals input into the overall op-amp circuit. If the impedance of the inverting branch of the op-amp circuit, for example, is different than the impedance of the local loop, the circuit will not cancel all of the local transmitted signal.
The impedance of the local loop contributes to the impedance of one branch of the amplifier circuit, and hence can affect the operation of the hybrid circuit. Thus, matching the impedance of the op-amp circuit impedance elements with the local loop impedance will allow the designer to cancel the local transmitted signal more completely.
Matching the impedance is made more difficult by several factors. One of these factors is the fact that the impedance of the local loop is not purely resistive. In practice, the local loop usually possesses the characteristics of a low-pass filter.
Another factor affecting impedance matching is that the transformers typically have non-ideal characteristics, and tend to have unpredictable impedances at low frequencies. One solution to this problem is to use better transformers, but these tend to be expensive and bulky.
Yet another problem with matching the impedance is the length of the local loop in the system, as the length of the loop affects the impedance. This is relatively easy to correct for if the length of the local loop is known.
One solution to this problem is made possible by the widespread use of data jacks, for example RJ 41S, RJ 45S or RJ 4MB jacks. These jacks include a resistor which a technician chooses upon installation of the jack based on the local loop loss, which can be translated into length. This resistor is known as the programming resistor. Although the primary function of the programming resistor is to provide an indication of loop loss in order to adjust transmitter output power for maximum dynamic range, it may also be used as a basis for switching the values of the op-amp impedances to better match them with the local loop impedance.
In these RJ 41S, RJ 45S and RJ 4MB jacks, the insulation between the pins can be insufficient to prevent high-voltage spikes from jumping to the programming resistor pins of the jack, and from there entering the modem. Thus, measuring the programming resistor by putting it into a low-voltage divider circuit, or using the programming resistor in the actual transmit circuit, can lead to damage of the modem circuitry.